1. Field of the Invention
This invention relates generally to charge-coupled devices and in particular to methods and apparatus for sensing and amplifying charge as it is emitted from CCD registers.
2. Description of the Prior Art
Charge-coupled devices (CCDs) may be used as storage devices in data processing (and optical) systems. A primary advantage which they offer is the potential for cost reduction in terms of cost per bit. In order to achieve this type of reduction, it is essential to have high densities within the CCDs. (CCD registers with high density capabilities are described in related applications Nos. 1 and 2.)
Higher density may be achieved by use of storage sites which occupy only small areas on a chip. This implies less charge storing capability within each site, and hence, less charge representative of bits of information. In other words, to achieve higher density, it is required to store and transport smaller quantities or packets of charge within the device. This has the effect of requiring a highly-sentitive detector or amplifier attached to the output terminal of the device so as to be able to detect these small charge packets.
It is further noted that the SPS register configurations described in the above-mentioned applications, Related Application Nos. 1 and 2, require only one amplifier per 256 storage sites. This high ratio of storage sites per amplifier is a measure of high density. Part of this savings in area required may be used for a better sense amplifier, which must be provided since the amount of charge is so small. Furthermore, packets of charge may be transferred up to 50 times between sense and amplification. Therefore, dissipation of charge is also a factor which must be accounted for in the design of the SPS registers and the charge detectors associated therewith.
Charge transported through the CCD corresponds to logical zeros or ones. Since the charge levels are so small, typically 50 to 230 .times. 10.sup.-.sup.15 Coulombs, the difference between the presence of a charge packet or no-charge, or a one and zero, is very small. Similarly, the signal to noise ratio within the CCD may be relatively low. Clearly, there is a need for a more sensitive detector as density increases.
Amplifiers are of course available in the prior art. One type may be termed "single-ended input." In such an amplifier, of critical importance is the pre-charge level. A pre-charge diode conditions the "single-ended input" terminal to a known voltage. After such conditioning, the diode turns off and the input terminal receives either a charge, which establishes a new level, or no-charge from the CCD. This new level is then used as an input to an inverter which acts as an amplifier. Such single-ended input devices are linear and have the advantage of being small. Hence, they are good for configurations such as the serpentine which require numerous amplifiers. However, their sensitivity is insufficient for use in the present purposes. Specifically, they cannot sense differences between the relatively small amounts of charge in question.
Accordingly, it is desirable to use the general principle of a differential amplifier, which is well known in the prior art. After two (opposing) nodes of the differential amplifier have been pre-charged to a known reference voltage, the input signal is delivered to a first node and a reference signal delivers a reference voltage to the second node. The imbalance between these two nodes is sensed and amplified. However, CCDs encounter special problems. If the input signal sense node is not isolated from the CCD register, and if it receives too much charge, then this sense node will act as an input signal from the amplifier to the CCD, i.e., it will inject charge back into the CCD register. Hence, the input node must be stabilized and isolated. Similarly, it is essential that the reference voltage be highly regulated. In the prior art, sense amplifiers used a reference voltage developed at one spot on the chip and distributed to the registers thereon. However, this reference voltage wanders with respect to the "1" and "0" and is not locked directly to variations in the injector circuit, i.e., variations in the amount of charge first injected into each charge packet. These variations put limits on the reference voltage method.